In vehicles on which driving power sources such as engines (internal combustion engines) are mounted, as transmissions to appropriately transmit torque and revolution speed generated by the engines to drive wheels in accordance with vehicle running conditions, automatic transmissions are known to automatically optimize a speed change ratio between the engines and the drive wheels.
Examples of the automatic transmissions mounted on vehicles include a planetary gear type transmission that sets up a gear stage using a clutch, a brake, and a planetary gear apparatus; and a continuously variable transmission (CVT: Continuously Variable Transmission) of belt type that adjusts a speed change ratio steplessly.
Further, examples of transmissions mounted on vehicles include an automated manual transmission (hereafter also referred to as AMT) in which actuators automatically carry out a speed change operation (change-over of gear stages). Specifically, in a constant mesh gear type transmission that includes a plurality of gear pairs (input shaft side gears and output shaft side gears), a synchromesh mechanism is actuated and controlled by a shift actuator and a selection actuator so as to switch each gear pair between a power transmitting state and a no-power transmitting state, thereby obtaining a desired speed change ratio (for example, see PLT 1). For coupling the AMT to the driving power source such as an engine, an automatic clutch is employed.
The synchromesh mechanism of the constant mesh gear type transmission includes a sleeve and a synchronizer ring. The sleeve is coupled to either an input shaft or an output shaft of the transmission and is shifted in the shift direction by the shift actuator. When the sleeve is shifted from a neutral position to a shift engagement position (gear engagement position), the sleeve is engaged with one gear among a plurality of gears supported in an idling state on either the input shaft or the output shaft, whereby the gear is engaged to either the input shaft or the output shaft (gear engagement). With the engagement of the gear, one gear pair turns into a power transmitting state, thereby obtaining a gear stage corresponding to the gear pair. Subsequently, when the sleeve is shifted, from the state (gear engagement state) in which the sleeve is engaged with the gear, in a direction opposite to the direction at the time of the shift engagement, then the sleeve is disengaged from the gear. As a result, the gear turns into an idling state and the sleeve turns into a neutral state. This operation of disengaging the sleeve from the engagement state is referred to as shift disengagement (gear disengagement).
The synchronizer ring is configured to synchronize the input shaft with the output shaft of the transmission with a frictional force that increases in accordance with the movement of the sleeve. This ensures that even if the input shaft does not synchronize with the output shaft of the transmission before speed change, synchronization occurs with the movement of the sleeve during the speed change, resulting in a smooth speed change.
The automatic clutch includes a frictional clutch and a clutch operating apparatus that operates the clutch. The clutch operating apparatus includes, for example, a release bearing, a release fork, and a hydraulic actuator to actuate the release fork, and the clutch operating apparatus is configured to control oil pressure of the actuator to automatically turn the clutch into a decoupled state or a coupled state (engaged state).
PLT 2, listed below, discloses a speed changing control apparatus of a vehicle that includes a mechanical clutch disposed between an engine and a gear type transmission, wherein after gear disengagement, the clutch is moved from a decoupled position to a position just before a half clutch region at the same time with gear engagement to a required gear stage, and then upon completion of the gear engagement, the clutch is moved to a completely-coupled position.
PLT 3 discloses a speed changing control apparatus wherein in the speed change of a double clutch, movement is permitted only as far as a point, within a range where the driving power is not transmitted, that becomes as close as possible to a point where the transmitting of driving power is stated, at the time of disengagement of an automatic clutch.
PLT 4 discloses a control method by which engine output is controlled so that a vehicle acceleration is approximately zero when a speed change is requested, and the control of the engine output is maintained for a predetermined period of time and then a clutch is completely released.    PTL 1: JP2008-202684A    PTL 2: JP2007-211945A    PTL 3: JP2003-112541A    PTL 4: JP11-173349A